A disease is defined as a disorder of function or structure. On the other hand, a medical condition simply reflects a state of health.
Infertility is caused by many factors. The risk of infertility increases with age. Thus, a woman who is infertile in her 40s may not have been infertile in her 20s. Sexually transmitted diseases, smoking and being overweight also affect fertility. In such cases, it is difficult to categorize infertility as a disease.
On the other hand, some cases of infertility may be caused by structural issues. For example, a man could have difficulty fathering a child if he has no vas deferens or a woman could find it difficult to conceive if she has severe endometriosis. In such cases, infertility can be categorized as a disease.